considered as a zoological Character. 453 



die doursucouli (Aotus trivirgatus), the only generic type 

 which I had not previously seen alive, has been delayed from 

 various causes; and that I was not aware that the primary 

 observation which it records had been previously and origi- 

 nally made by Azara, till the moment of commencing it. 



It will have been remarked, no doubt, in the second extract 

 which I have given from the work of Azara, that this acute 

 zoologist speaks in more qualified terms of the unopposable 

 power of the anterior thumb in the cay, than in either of the 

 other two species which he has described; and in this respect 

 his observations are in perfect accordance with my own. Of 

 the eight natural genera which include all the known Simiadae 

 of the Western Hemisphere, and of which I have examined 

 many living species, besides the skins of a much greater 

 number, five have the anterior thumbs placed absolutely on 

 the same line with the other fingers, of the same form, acting 

 invariably in the same direction, and totally incapable of being 

 opposed to them : these are, Mycetes, Lagothrix, Aotus, 

 Pithecia, and Hapale. It is scarcely necessary to observe, 

 that the thumb is entirely wanting in A'teles, or exists only in 

 a rudimentary form beneath the skin ; and there remain only 

 the genera Cebus and Callithrix, in which I shall now proceed 

 to explain the modifications of form and function presented 

 by this organ. 



In describing the Cebus capucinus, Azara observes: — "II 

 y a cinq doigts a chaque pied, presque de la meme forme que 

 ceux de la main de Phomme, quoique le pouce de devant soit 

 moins separe que dans celui-ci, et qu'il ne soit pas plus gros 

 que les autres doigts." This passage exactly describes the 

 appearance of the organ in question, both in the genera Cebus 

 and Callithrix. The thumb of these animals is, in fact, placed 

 farther back than the general line of the other fingers, and, 

 on that account, when superficially noticed, has the semblance 

 of being opposed to them ; but, as Azara very correctly ob- 

 serves, it is less separated than in man ; and, I may add, than 

 in the true Simiae : it is, besides, of precisely the same long 

 slender form, is weaker than the rest, absolutely without power 

 of opposition, and habitually acts in the same direction. 1 

 have no doubt that it is the backward position of this organ 

 in the Cebi, and the deceptive appearance of opposition which 

 this circumstance produces, that has hitherto prevented zoo- 

 logists from observing the true characters and affinities of the 

 American Simiadae ; for, of all these mammals, the Cebi are 

 most commonly brought to Europe, and, unless examined with 

 this express view, previously suggested by the observation of 

 other genera, in which the character is less equivocally deve- 



LL 3 



